1. Field of the Invention
The present invention pertains to electronic devices that employ groups of capacitors to provide DC voltages, such as power electronic devices (e.g., a multi-level power inverter), and, in particular, to a method and circuit for balancing the charge on multiple capacitors of a capacitor bank of such an electronic device.
2. Description of the Related Art
A multilevel power inverter is a power electronic device that is structured to produce AC waveforms from one or more DC input voltages. Multilevel power inverters are used in a wide variety of applications, such as, without limitation, variable speed motor drives and as an interface between a high voltage DC transmission line and an AC transmission line.
The general concept behind a multilevel power inverter is to use a number of power semiconductor switches coupled to a number of lower level DC voltage sources to perform power conversion by synthesizing a staircase voltage waveform. A number of different topologies for implementing a multilevel power inverter are well known, including but not limited to the neutral point clamped (NPC) topology and the H-bridge topology.
As is known in the art, a bank of capacitors (the “DC link”) coupled to one or more DC voltage inputs is often used to provide the multiple DC voltage sources required for operation of a multilevel power inverter. For example, it is known to use such a DC link comprising a bank of capacitors in the NPC topology described above. Under normal, sinusoidal operation, the DC link capacitor bank voltages of a multilevel inverter with more than three levels will tend to become unbalanced. As is known, operation of a multilevel power inverter with unbalanced voltages in the DC link capacitor bank will adversely affect the performance of the multilevel power inverter due to the generation of uncharacteristic harmonics in the inverter output voltage and the presence of overvoltage across the semiconductor switches.
One known approach to the capacitor balancing problem is to apply a specialized, multi-secondary winding transformer that inherently enforces capacitor voltage balancing across all power converter levels. A second known approach is to apply advanced control techniques to the load current in order to manage the energy flow in and out of the DC link capacitor banks. These solutions, however, have proven to be cost prohibitive and/or functionally inadequate in many applications.